It was one of the brightest nova events of the decade and clearly visible to the naked eye, yet no one but the XMM-Newton space telescope was there for the party.

The event was captured serendipitously by XMM as it traversed from one target to another in a routine slew survey last October. The clue that something unusual was going on came in the shape of a blinding source of X-rays that XMM detected at a rate of around 50 hits a second.

The nova V598 was accidentally discovered by XMM as part of a routine slew survey. The contours in the image indicate the intensity of the X-rays, and are overlaid on a colour composite of the nova from the SuperCOSMOS Sky Surveys (SSS), Royal Observatory, Edinburgh. Image: Contours: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.), Background: SSS.

The only object located at the mysterious source of the X-rays was a star known solely by its catalogue number USNO-A2.0 0450-03360039. A notification was immediately circulated across the internet to inform other astronomers of the X-ray source, and thanks to quick follow up observations by astronomers using the 6.5-metre Magellan-Clay telescope at Las Campanas Observatory in Chile, USNO-A2.0 0450-03360039 was found to have dramatically brightened by a factor of 600, allowing the object to be classified as a nova.

Novae occur when a white dwarf star has consumed enough gas from a nearby companion star to trigger a nuclear reaction. This releases huge quantities of energy, lighting the star up like a beacon. Curiously, the detection of X-rays implied that the explosion had occurred some time before XMM arrived on the scene, since X-rays are only released after the expanding cloud of dust and debris from the initial explosion has dispersed.

Since dedicated amateur and professional astronomers regularly find novae by sweeping the night sky for stars that suddenly brighten, it was surprising that no reports of the brightening had been announced. Richard Saxton of ESA’s European Space Astronomy Centre in Spain contacted the All Sky Automated Survey, a project dedicated to monitoring stars brighter than 14th magnitude all over the sky, and asked them to check their data. They found the nova, and pinpointed its detonation to the 5 June 2007. It would have been clearly visible, even to the unaided eye.

An optical image (left) of the nova, compared with an X-ray image (right), which reveals the extend of the nova explosion. Image: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.)

"Anyone who went outside that night and looked towards the
constellation of Puppis would have seen it," says Saxton.

The nova is now been officially renamed to the more manageable V598 Puppis and has been confirmed as one of the brightest nova events for almost a decade, doubling the irony that it was not spotted by anyone during its brilliant peak. But as news of the discovery spread, the global effort to track its fading light intensified.

"Suddenly there was all this data being collected about the star,” says Andy Read of the University of Leceister. “For variable star work like this, the contribution of the amateur community can be at least as important as that from the professionals."

While an important discovery, the chance observation by XMM raises the question as to what other celestial wonders are acting out without an audience.

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